Late cretaceous magmatism in the Northern Lhasa subterrane, central Tibet: geodynamic implications

• Published in: International geology review Vol. 65; no. 10; pp. 1644 - 1663
• Main Authors: Sun, Miao, Chen, Wei, Klemd, Reiner, Tang, Ju-Xing, Ma, Xu-Dong, Liu, Qing-Ping
• Format: Journal Article
• Language: English
• Published: Philadelphia Taylor & Francis 31-05-2023; Taylor & Francis Ltd
• Subjects: Anomalies; Central Tibet; Change of composition; Composition; Convection; Convective removal; Cretaceous; Evolution; Fluids; Garnet; Geochronology; High temperature; Isotopes; Juveniles; Late Cretaceous; Lava; Low temperature; Low-angle subduction; Magma; Mantle convection; Melting; Neo-Tethys oceanic plate; Plateaus; Potassium oxides; Ratios; Rock; Rocks; Strontium 87; Strontium isotopes; Subduction; Subduction (geology); Tectonics; Zircon
• ISSN: 0020-6814; 1938-2839
• DOI: 10.1080/00206814.2022.2101150

Compositional changes of successively emplaced magmatites can be used to infer changes in their petrogenetic evolution and geodynamic setting. The central-northern Lhasa terrane hosts extensive Late Cretaceous magmatism, however, the mechanism that triggered the magmatism and its relationship to the directly preceding Late Cretaceous tectonic transition in the central Tibetan Plateau is still unresolved. Therefore, we conducted a detailed petrological, geochronological, geochemical, and isotopic study on recently identified Late Cretaceous doleritic and granodioritic rocks in the Xiongmei area, northern Lhasa subterrane. The potassic (K
2
O/Na
2
O ratios >1) dolerites (ca. 80 Ma) display a post-collisional basaltic shoshonitic signature with uniform moderately enriched whole-rock Sr-Nd and zircon-Hf isotopic compositions ((
87
Sr/
86
Sr)
t
 = 0.7070 to 0.7095, whole-rock ε
Nd
(t) = - 2.4 to - 1.3, and zircon ε
Hf
(t) = - 5.9 to +1.9), suggesting that they evolved from a similar parental magma and were likely derived by partial melting of an enriched lithospheric mantle metasomatized by hydrous fluids. The granodiorites are geochemically divided into medium-K and high-K calc-alkaline rocks. The medium-K samples (ca. 80 Ma) are generally characterized by low K
2
O/Na
2
O ratios <1, whereas the high-K samples (78-76 Ma) display high K
2
O/Na
2
O ratios >1. The medium-K samples have positive zircon ε
Hf
(t) values from +4.8 to +6.5 and uniform whole-rock ɛ
Nd
(t) values from - 2.4 to - 1.3, which significantly differ from those of the high-K samples that have negative to positive zircon ε
Hf
(t) values from - 1.9 to +2.8 and negative whole-rock ɛ
Nd
(t) values from - 8.4 to - 5.1. The medium-K samples are characterized by high Sr contents and Sr/Y ratios, coupled with low Y and HREE contents, similar to adakitic compositions, and were interpreted to have been derived from deep partial melting of the foundering juvenile garnet-bearing mafic lower crust at relatively low temperatures. Compared to the medium-K samples, the high-K samples have lower Sr/Y and (La/Yb)
N
ratios, coupled with flat HREE patterns and distinct negative Eu, Sr and Ti anomalies and are most likely derived from shallower partial melting of the K-rich juvenile lower crust at relatively high temperatures. Considering the occurrence of variable magmatism in the central-northern Lhasa subterrane during the Late Cretaceous, the change in magma composition and lower crustal melting conditions (cold-deep vs. hot-shallow) suggest that the lithospheric mantle keel was partially detached by small-scale mantle convection in response to the low-angle Neo-Tethys-associated subduction between 85 and 70 Ma.